If both walls are vertical:
The best way to do it is to draw the lower wall shorter by the the thickness of the taller wall and their reference lines do not touch each other. Only then the 2d representation will look right in situation where both the wall are below cut plane.
IF one of the walls is slanted or profile:
Do as discribed in the above posts.
Again, inconsistencies in AC.......
Howard Phua
Win 10, Archicad 19 INT